Theoretically, there should be no need for shaft anodes if the shafts are properly isolated inside the boat. But I always kept one on each shaft because I was never sure of the isolation. I probably overworried because the anodes never wore down. If I were still boating, I'd keep them - hjust in case- There is no downside to doing so.
BTW, I still have a pair (I think) and would make them avail for half the West Marine price + shipping. 1 1/4" dia